1. Field of the Invention
The present invention relates to a DC electromagnet apparatus used for driving an electromagnetic contactor or the like.
2. Description of Related Art
Generally, an electromagnet has a stator core on which an operation coil is wound, and a movable core faced with the stator core via a gap. When the stator core is energized, the movable core is attracted to the stator core travels the length of the gap. During this movement, the movable core moves against the force caused by a load to be driven and a spring. In this case, a larger attractive force is required at an initial stage of closing the electromagnet, and a smaller attractive force is enough to maintain the movable core at the closing position after completing the attraction.
As a conventional electromagnet apparatus implemented in view of such characteristics, an electromagnet apparatus is known using a driving circuit as described in Japanese Patent Application Laying-Open No. 168607/1984.
FIG. 1 shows the conventional driving circuit for an electromagnet. In this figure, an operation coil 1 of the electromagnet is connected in series with a switching device 5d such as a transistor between the output terminals P and N of a DC power supply. The switching device 5d is controlled by a control power supply circuit 6, a voltage detecting circuit 7, a timer circuit 8 and an oscillating circuit 9. The outputs of the voltage detecting circuit 7 and the timer circuit 8, and the output of the oscillating circuit 9 are supplied to the switching device 5d through an OR gate 10 and a resistor 11. In addition, a manual switch 4 is provided for switching the DC power supply.
The operation of the circuit of FIG. 1 will be explained referring to a waveform diagram of FIG. 2. When the manual switch 4 is turned on, and the power supply voltage across the terminals P and N exceeds a predetermined value, the voltage detecting circuit 7 produces an output signal, and supplies it to the switching device 5d through the OR gate 10 and the resistor 11. Thus, the switching device 5d is turned on at time t1 of FIG. 2, thereby supplying the operation coil 1 with a large current required to operate the electromagnet. The current causes the electromagnet to close at time t.sub.2 of FIG. 2. At this point, the switching device 5d is still conductive, and hence a large current flows through the operation coil 1. The output of the voltage detecting circuit 7 starts the timer circuit 8. The timer circuit 8 outputs a signal after a predetermined time period, and stops the signal from the voltage detecting circuit 7, thereby turning off the switching device 5d at time t.sub.3. At the same time, the output of the timer circuit 8 is supplied to the oscillating circuit 9. The oscillating circuit starts to operate, and outputs a pulse train. The pulse train is applied to the switching device 5d through the OR gate 10 and the resistor 11, and the switching device 5d turns on and off alternately. Accordingly, the operation coil 1 is supplied with a pulsatile voltage. In this case, the actual current flowing through the operation coil 1 is smoothed by a free-wheeling diode 14. Thus, the electromagnet is maintained at a making (closing) condition with a small current by selecting an appropriate ON/OFF ratio.
The conventional DC electromagnet apparatus, however, presents a problem, which will be explained with reference to FIG. 3. In the circuit shown in FIG. 1, since the power supply voltage is directly applied to the operation coil 1, the attractive force of the electromagnet will change greatly depending on the power supply voltage. Currently, the range of a working voltage for driving an electromagnetic contactor is specified at 85-110% of its rated voltage. Therefore, the electromagnetic portion of the contactor must be designed such that it produces, at a making operation, a sufficient attractive force even if a voltage of 85% of the rated voltage is used. The attractive force f of an electromagnet, however, is proportional to the square of the applied voltage v as show in FIG. 3. As a result, an increasing voltage will produce an unduly large attractive force. This large attractive force will produce a strong impact on the core of the electromagnet and other portions thereof, and hence, will shorten a lifetime of the mechanism of the contactor. In addition, the large attractive force will cause chattering of the main contacts, and this will reduce the lifetime of the contacts.
FIG. 3 is a graph illustrating the attractive force of a common electromagnet. The abscissa represents an applied voltage v and the ordinate represents the attractive force f of the electromagnet, and f.sub.0 denotes the attractive force required to close the electromagnet. It is seen from this graph that the attractive force f sharply increases from the attractive force f0 as the applied voltage v increases.
To overcome this problem, another DC electromagnet apparatus is proposed by Japanese Patent Application Laying-Open No. 187304/1986. FIG. 4 shows the circuit, and FIGS. 5(a) and 5(b) show the relationship between the changes in the coil current flowing through a coil 1 of FIG. 4, and the output of a constant current circuit 16. More specifically, the electromagnet driving circuit of FIG. 4 includes a control power supply circuit 6, a voltage detecting circuit 7, a timer circuit 15, a constant current circuit 16 and an AND gate 17. The control power supply circuit supplies power to the voltage detecting circuit 7, timer circuit 15 and constant current circuit 16. A first output of the voltage detecting circuit 7 is connected to the input of the timer circuit 15 and a second output to a first input of the AND gate 17. The output of the timer circuit 15 is connected to a first input A of the constant current circuit 16, and the output of the constant current circuit 16 is connected to a second input of the AND gate.
The output of the AND gate 17 is connected through a resistor 11 to the base of a transistor 5b having its collector connected through a resistor 13 to the base of a transistor 5a. The transistors 5a, 5b and the resistors 11, 13 comprise a switching circuit 5.
The collector of the transistor 5a is connected to the control power supply circuit 6, and its emitter to an operation coil 1 of an electromagnet in series with a current detecting circuit 12 comprising a resistor 12a. The junction of the operation coil 1 and the resistor 12a is coupled to an input B of the constant current circuit 16.
The operation of the circuit is as follows: First, coil current i flowing through the operation coil 1 produces a voltage across resistor 12a which is coupled to the input B of constant current circuit 16. If the coil current i is smaller than a set level I.sub.1 of the constant current circuit 16, the constant current circuit 16 produces an output so that a voltage is supplied via AND gate 17 and switching circuit 5 to the operation coil 1 to increase the coil current i. In contrast, if the coil current i exceeds the set level I.sub.1, the constant current circuit 16 stops the output so that the coil current i is reduced. If the coil current i become less than the set level I.sub.1 again, the constant current circuit 16 restarts the output in order to increase the coil current i. This operation is repeated to maintain the coil current i at a constant value. Furthermore, the holding current of the electromagnet is produced by performing a similar control using the set level I.sub.2 lower than the set level I.sub.1 so that the coil current i required to hold the electromagnet is maintained.
This conventional DC electromagnet apparatus presents the following problem: FIGS. 6(a), 6(b) and 6(c) are a waveform diagrams illustrating the operation of the electromagnet apparatus at a low power supply voltage v1 and at a high power supply voltage v.sub.2. When the power supply voltage v is outputted at time t.sub.0 as shown in FIG. 6(a), the coil current i will start to increase. In this case, the rising rate of the coil current i is small as indicated by the solid line i.sub.1 when the power supply voltage v is at the low voltage v.sub.2, whereas it is large as indicated by the broken line i.sub.2 when the power supply voltage v is at the high voltage v.sub.2. As a result, even if the coil current i is limited to the set value I.sub.1 as shown in FIG. 6(b), the electromagnet will make (close) at time t.sub.1 after a rather long operating time TM.sub.1 at the low power supply voltage v.sub.1, and at time t.sub.2 after a short operating time TM.sub.2 at the high power supply voltage v.sub.2. In FIG. 6(b), the dip points of the coil current i correspond to the making (closing) points of the electromagnet. The dip points are induced by an increase in the inductance of the operation coil 1 when the electromagnet is closed. This circuit must provide the operation coil 1 with a coil current throughout the time period TC (until time t.sub.3) that exceeds the longer operating time TM.sub.1 associated with the lower power supply voltage v.sub.1, even if the higher power supply voltage v.sub.2 is supplied, and hence the electromagnet completes its making in the shorter operating time TM.sub.2. As a result, when the power supply voltage v is v.sub.2 , an excessive coil current flows, which makes it difficult to reduce the impact at the making of the electromagnet.